1. Field of the Invention
This invention relates to an optical disc and a signal reproduction apparatus for the same. More particularly, this invention relates to an optical disc with which crosstalk signal can be cancelled, utilizing read out signals of a plurality of tracks, to obtain recorded information of desired track, and a signal processing circuit for processing read-out signals from the optical disc.
2. Description of the Prior Art
There is known an optical disc on which signal is recorded by spirally forming signal pit array serving as recording track on a signal recording surface of the disc from inner circumferential side to outer circumferential side thereof. In order to improve recording density and capacity of such an optical disc, there have been proposed various methods such as increasing recording density in recording track direction, reducing a diameter of read-out laser light or narrowing recording track pitch.
By the method of narrowing recording track pitch, minimization of the track pitch is limited by the size of light spot converged on the disc surface. If the track pitch is narrowed without improvement of the size of the light spot, recorded signal of target track may be reproduced together with recorded signals of neighboring tracks thereof, and the read-out signal may include large crosstalk. Therefore, recorded signal of the target track cannot be correctly obtained. As one of the countermeasures for the crosstalk problem, there is proposed a method of irradiating three light beams on three neighboring recording tracks, respectively, reading out signals from the three recording tracks and eliminating crosstalk mixed in the center of the three recording tracks utilizing read-out signals of the two neighboring tracks on both sides of the center track. In a method proposed in Japanese Patent Application Laid-Open No. Hei:3-40225, variable-frequency filters are provided and filter coefficients prescribing frequency characteristics of the filters are varied so that read-out signal of the center track does not include crosstalk signal, thereby cancelling crosstalk components. Another proposal teaches calculating cancellation coefficients so that read-out signals of neighboring tracks do not have correlation with each other.
However, according to the above-mentioned techniques, calculation of optimum filter coefficients takes a certain period of time, and hence it is difficult to rapidly follow up the variation of disc condition. In addition, those techniques cannot eliminate affection by asymmetrical aberration (coma-aberration) of lens-system in reproduction apparatus or curvature of optical disc, and therefore crosstalk cannot be sufficiently reduced.
On the other hand, in reproduction of signal from an optical disc, it is necessary to adjust, in phase, clock signal of read-out signal to a reference clock produced by the reproduction apparatus. In a general optical disc, clock-pits CP are formed on the recording surface, as illustrated in FIG. 1A, and the timings of clock-pits CP are detected as peak points in waveform of reproduced signal. Then, the detected timings of the clock-pits CP are compared with timings of the reference clock to obtain phase error of the reference clock with respect to the reproduced signal. However, when rotation of spindle motor in an apparatus used for original recording of optical disc is irregular, the clock-pits CP are formed out of alignment in radial direction of the disc as shown in FIG. 1B, although they should be formed in alignment as shown in FIG. 1A. When this phenomenon takes place in an optical disc having narrowed track pitch, clock-pit signal of neighboring tracks are mixed into the reproduction signal of target track at shifted timings due to large crosstalk, and therefore phase error of the reference clock with respect to the reproduced clock signal cannot be correctly detected.